Can woodchip bioreactors be used at a catchment scale? Nitrate performance and sediment considerations

Authors: Feyereisen, Gary; GHANE, E - Michigan State University; Schumacher, Todd; Dalzell, Brent; Williams, Mark

Submitted to: Journal of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2023
Publication Date: 4/3/2023
Citation: Feyereisen, G.W., Ghane, E., Schumacher, T.W., Dalzell, B.J., Williams, M.R. 2023. Can woodchip bioreactors be used at a catchment scale? Nitrate performance and sediment considerations. Journal of the ASABE. 66(2):367-379. https://doi.org/10.13031/ja.15496.
DOI: https://doi.org/10.13031/ja.15496

Interpretive Summary: Woodchip bioreactor is a conservation practice deployed at the field scale to remove nitrate from agricultural tile drainage. Installing bioreactors at a multi-field, small watershed scale could provide benefits such as more cost-effective treatment, centralized management, and ease of verifying performance. This study evaluated the nitrate removal performance of a unique three-bed bioreactor with cascading inlets and describes how sedimentation issues were addressed. The bioreactor system, located in Faribault County, MN, treated a tile-drained 615-acre watershed in corn and soybean production. Over one drainage season, the bioreactors treated 55% of the watershed discharge and removed 12.5% of the nitrate load during twelve runoff events. Nitrate removal rates were similar to those for field-scale woodchip bioreactors. There were five additional events during which flow was impeded by sediment. Sampling along one of the beds indicated that on the order of 5 tons of sediment were trapped. To solve the problem and prolong bioreactor lifespan, the researchers designed and installed a system to monitor turbidity and close off flow when necessary. In conclusion, bioreactors can be used at small catchment scale, but provisions may be needed to avoid sediment problems. This research demonstrates how bioreactors can be used at a small watershed scale and is pertinent to engineers and practitioners, researchers and academics, and agency personnel with interest in using denitrifying bioreactors to remove nitrate-N from water.

Technical Abstract: Eutrophication causes increased growth of algae and nuisance aquatic plants that interfere with beneficial use of water resources. Denitrifying bioreactors, a structural practice deployed at the field scale to meet water quality goals, have been underutilized and require additional evaluation at the small catchment scale. The objective of this study was to quantify performance of a large, multi-bed denitrifying bioreactor system sized to treat agricultural drainage runoff (combined drainage discharge and surface runoff) from a 249-ha catchment. Three woodchip bioreactor beds, 7.6 m wide by 41 m long by 1.5 m deep, with cascading inlets, were constructed in 2016 in southern Minnesota, U.S. The beds received runoff for one water year from a catchment area that is 91% tile-drained row crops, primarily maize and soybeans. Initial woodchip quality differed among the three beds, affecting flow and nitrate removal rates. Bioreactor flow was unimpeded by sediment for twelve events from September 2016 to July 2017 during which time 55% of the discharge from the catchment was treated in the bioreactor beds. Average daily nitrate removal rates ranged from 2.5 to 6.5 g-N m-3 d-1 for the three bioreactor beds, with nitrate-N load removal of flow through the beds between 19 and 27%. When accounting for untreated by-pass flow, overall nitrate-N removal of the multi-bed system was 12.5%. During high flow events, incoming sediment clogged the reactor beds, decreasing their performance. There were 4,520 kg of sediment trapped in one bed with evidence the other two trapped a similar load. To solve this problem and prolong the bioreactor lifespan, we installed a shutoff gate that activated when inflow turbidity exceeded a threshold value. In conclusion, findings suggest that catchment-scale denitrifying bioreactors can successfully remove nitrate load from agricultural runoff, but sediment-preventing measures may be needed to extend the lifespan of the bioreactor.